Secondary prevention of stroke has been demonstrated efficacious in numerous large scale randomized clinical trials (RCT's). Despite the large number of RCT's, few have included elderly people. This lack of research on secondary stroke prevention in the elderly seems to be paradoxical as the incidence of stroke increases with increasing age with some evidence of a worsening prognosis as well. As RCT's often exclude those at greatest risk, well-designed observational studies can complement RCT's by providing information to further understand both the beneficial, as well as unintended effects of secondary drug prevention of stroke. Yet, confounding introduced by non-random allocation to drug treatment, defining of exposures when treatment may vary through time and bias introduced with informative censoring pose methodological challenges. In the nursing home setting these challenges are compounded since both within facility clustering of residents and the interrelation between organizational factors and hospitalizations occur. The overall goal of this study is to quantify the beneficial and unintended effects of drug therapy used in the secondary prevention of ischemic stroke in an elderly nursing home population. Of particular concern are the differences in treatment patterns and how they effect both mortality and rates of subsequent stroke. Using data collected from the Health Care Financing Administration (HCFA) Multistate Nursing Home Case-mix and Quality Demonstration Project, newly admitted nursing home residents in 5 states (ME, MS, KS, NY and SD) from the years 1992 to 1996 will be identified. Residents who bare at least 65 years of age and have had an ischemic stroke within the 90 days prior to admission will be eligible for inclusion. The specific aims of this study are 1) to quantify the effects of antiplatelet therapy on the secondary prevention of ischemic stroke, 2) to determine the extent to which antiplatelet therapy increases the risk of unintended adverse effects and 3) to compare methods to analytically control for confounding in observational pharmacoepidemiology studies and further determine their differential effects on precision.